Golf balls of various structures have been proposed such as one-piece golf balls which consist of a golf ball body; two-piece golf balls which include a core and a cover; three-piece golf balls which include a core having a center and a single intermediate layer covering the center, and a cover covering the core; and multi-piece golf balls which include a core having a center and two or more intermediate layers covering the center, and a cover covering the core.
Widely used materials for golf balls are ionomer resins because such resins provide golf balls which have high rigidity and which fly a long distance; in particular, highly resilient materials are desired. Ionomer resins having an increased degree of neutralization are known to enhance resilience. They, however, tend to increase the hardness simultaneously, which results in lower flexibility and a poorer shot feeling.
In this respect, a method of decreasing the hardness while enhancing resilience is proposed. In this method, a large amount of a fatty acid (metallic soap) is added to a highly neutralized ionomer resin; however, the acid component in the fatty acid consumes metal ions used for neutralization, and therefore the effect of enhancing resilience due to a high degree of neutralization is not sufficiently achieved. Thus, the method has an insufficient effect in enhancing flexibility and resilience in order to provide a golf ball that simultaneously achieves a good shot feeling and resilience. In addition, the method requires a large amount of the metal component.
For example, Patent Literature 1 discloses a golf ball material prepared using an ionomer resin and a compound that contains both acid and amine functional groups in the same molecule. This golf ball material, however, should still be improved from the viewpoint of simultaneous enhancement of flexibility and resilience.